1. Field of the Invention
The present invention relates to an image forming apparatus that forms a color image through the steps of forming latent images by applying laser beams to photosensitive drums via polygon mirrors for two or more predetermined number of colors (e.g., four colors including magenta (M), cyan (C), yellow (Y) and black (K)), forming toner images corresponding to the formed latent images, and superposing the toner images of the predetermined number of colors to be transferred to a paper sheet. In particular, the present invention relates to a color copier, a color printer and a multifunction apparatus having either one of these functions.
2. Description of Related Art
Recently, as for the image forming apparatus such as a copier or a printer that forms a color image through the steps of forming latent images by applying laser beams to photosensitive drums via polygon mirrors for two or more predetermined number of colors (e.g., four colors including magenta (M), cyan (C), yellow (Y) and black (K)), forming toner images corresponding to the formed latent images, and superposing the toner images of the predetermined number of colors to be transferred to a paper sheet (i.e., through a so-called electrophotography process), it becomes a standby state for saving power consumption if a predetermined time period (e.g., a minute) passes without any input such as an external operation.
In this standby state or other states except a printing operation state, polygon motors that drive polygon mirrors are not driven. For this reason, when the apparatus returns from the standby state to an operating state, “rotation phases” of the polygon mirrors corresponding to individual colors may be shifted (i.e., a phase difference is generated) resulting in occurrence of color drift. Therefore, in order to prevent the occurrence of color drift, drive control of the polygon motors should be performed so that two or more predetermined number (e.g., four colors) of rotation phases of the polygon mirrors match each other.
Note that a rotation angle of a direction of a mirror surface of the polygon mirror with respect to a reference direction (e.g., a direction of the perpendicular from the center of the rotation axis of the polygon mirror to the axis of the photosensitive drum) is referred to as the “rotation phase” here. For example, in a case of a polygon mirror having six mirror surfaces, a rotation phase difference between polygon mirrors is within a range of −30 to +30 degrees.
A printer that can make rotation phases of the polygon mirrors match each other includes laser beam detectors for detecting laser beams scanning by the polygon mirrors corresponding to magenta (M), cyan (C), yellow (Y) and black (K) colors at predetermined positions on scanning paths as disclosed in JP-A-H9-233281 for example, and with reference to output signals of the laser beam detectors, rotation phase differences (i.e., phase differences) of all polygon mirrors (e.g., three polygon mirrors of magenta (M), cyan (C) and yellow (Y)) except one polygon mirror as a reference (e.g., a polygon mirror corresponding to black (K)) are calculated with respect to the reference polygon mirror, so that rotation frequencies of the polygon motors corresponding to the three polygon mirrors are changed for predetermined time periods.
Here, one polygon mirror (e.g., a polygon mirror corresponding to black (K)) is regarded as a reference, and the rotation frequency of the polygon motor that drives the other polygon mirror (e.g., a polygon mirror corresponding to magenta (M)) is changed based on the rotation phase difference (i.e., phase difference) of the other polygon mirror (e.g., the polygon mirror corresponding to magenta (M)) in the conventional image forming apparatus such as a printer described above. Therefore, if the rotation phase difference (i.e., phase difference) is large, time period necessary for making the rotation phases match each other (here referred to as phase control time) increases. In addition, if change quantity of the rotation frequency of the polygon motor is increased for decreasing the phase control time, overshoot or undershoot of the rotation frequency of the polygon motor occurs resulting in a difficulty in making the rotation phases match each other.